Lithography apparatuses are used, for example, in the production of integrated circuits (ICs) for imaging a mask pattern in a mask onto a substrate such as e.g. a silicon wafer. In the process, a light beam produced by an illumination apparatus is directed to the substrate through the mask. An exposure lens, consisting of a plurality of optical elements, serves to focus the light beam on the substrate.
The extent of the smallest structure elements that can be realized on the wafer is proportional to the wavelength of the light utilized for the exposure, and inversely proportional to the numerical aperture of the optical elements (lens elements or mirrors) utilized for beam shaping. In order to meet the requirements of ever smaller structures, a development focused on light sources with ever shorter wavelengths, which development culminated in EUV (extreme ultraviolet) light sources with a wavelength of between 5 nm and 30 nm (e.g. 13 nm). Such low wavelengths enable imaging of the smallest structures on the wafer. Since light in this wavelength range is absorbed by atmospheric gases, the beam path of such EUV lithography apparatuses is situated in a high vacuum. Furthermore, there is no material sufficiently transparent in the aforementioned wavelength range, which is why use is made of mirrors as optical elements for shaping and guiding the EUV radiation. The second precondition for small structure dimensions, namely a high numerical aperture, means that the employed mirrors have to be very large and near-wafer mirrors may have a diameter of e.g. 300 to 500 mm or more. Such large mirrors generally have a relatively large mass, which in turn places increased constraints on a low-deformation mount and actuation.
An option for mounting or actuating optical elements, such as e.g. mirrors in a lithography apparatus, lies in mounting via piezoelectric actuators. Examples therefor are disclosed in US2004/0212794A1, EP 1 879 218 A1 and US2003/0234989A1.
Document US2004/257549A1 describes a lithographic apparatus, a projection system, a method of projecting and a device manufacturing method. The projection system includes at least one projection device configured to receive a beam of radiation coming from a first object and project the beam to a second object. The projection system further includes a sensor configured to measure a spatial orientation of the at least one projection device and a processing unit configured to communicate with the at least one sensor. The processing unit is configured to communicate with a positioning device configured to adjust the position of at least one of the first object and the second object based on the measured spatial orientation of the at least one projection device.
Moreover, document US2004/227107A1 shows a lithographic apparatus and a manufacturing method. Further, US2004/227107A1 describes that, in a projection system for EUV, the positions of mirrors are measured and controlled relative to each other, rather than to a reference frame. Relative position measurements may be made by interferometers or capacitive sensors mounted on rigid extensions of the mirrors.
However, systems with piezoelectric actuators are reaching their limits in view of the increasing demands when positioning. In order to meet these increasing demands, holders or mounts with Lorentz actuators have been proposed, in which plunger coils are employed as drive for correcting the location of optical elements. In addition to more precise positioning, such Lorentz actuators also have various other advantages over piezoelectric actuators, such as e.g. lower rigidity, greater robustness against environmental influences and smaller parasitic effects. However, mounting the optical elements by way of Lorentz actuators is linked to a greater complexity of the system, since this is an active mount, in which the location of the optical elements is continuously corrected.